Scale book-covers proportional to the full screen of your e-reader.
The calibre cover-grid looks even better with all cover-images in the same size.

Scaling book-covers proportional usually adds an empty margin.
With „Prettify Cover“ you can choose how to align the image and how to fill the margin:

Black

White

Custom color

Cut-out method:
(Scaling the smallest side of the image to fit the e-reader full screen.)

Content-aware background (optional)
(Only available when GIMP and the GIMP Plug-in „resynthesizer“ is installed)

Usage:

Add „Prettify Cover“ to your toolbar. (Preferences > Toolbars. )

Click the arrow next to the „Prettify Cover“ icon in the toolbar and set the default resolution.

Select one or more books and click the „Prettify Cover“ icon

Choose the best cover by clicking the image

Now you've generated the new book-cover for calibre, only.

If you want this new cover in your ebook (epub, etc.) use the plug-in „Polish Book“ and make sure the option „update the cover in the book files“ is checked.
Click OK and upload the ebook with the new cover to your e-reader.

Content-aware background:

You can use „Prettify Cover“ without this feature.
If you want to use it you need GIMP and the resynthesizer plug-in for GIMP installed.

Version 0.9.3 - 2015-12-17
- added color palette widget with a colorpicker.
"most used color" groupbox is renamed to "custom color"
on start it's still set to the most used color.
open the widget by clicking the button labeled "change color"
- alignment buttons show current state
- timeout period for content-aware background rendering has been increased
from 99 to 120sec (some covers took more than 99sec on my end)
- moved help button below content-aware options
- updated help
- optimized code

Version 0.9.2 - 2015-12-07
- fixed the content-aware bg. preview.
it didn't match the final result because a thumbnail was rendererd for the preview.
now the full-size cover is processed for the preview and scaled down to thumbnail size.
the preview operation takes longer (approx. 30sec) but there is no more rendering done when choosing the cover.
- reduced thumbnail height. dialog was to large and some btns unavailable for monitors with 768px height
- Cut-out image preview starts centered
- updated german and english help files
- abbreviated, renamed and simplified text labels to better fit in UI
- optimized code

Thanks for your bugreport.
I will upgrade "prettify cover" for calibre version 3.x in the next 3-4 weeks. For the time beeing downgrade to calibre 2.8.5 (last 2.x version) if you want to use the gimp features... (afaik all features are working in 3.x only the gimp features don't)